A seismic operation to establish a seismographic survey is characterised geometrically by position parameters of the source and of the receiver. These parameters are four in number in the case of a conventional operation carried out in the plane of the earth's surface. From the signals received by the receiver, an echo is obtained which is a function of time and position parameters relating to the source-receiver, variables x,y, depth z, as a function of the different geological strata.
Generally, a strong redundancy of instantaneous measurements is thus obtained which can be used in accordance with the method known under the name multiple covering. The multiple covering method permits focussing the reception of signals on predetermined geological levels or strata and it is thus possible to reduce the level of undesirable signals with respect to actual reflections. The multiple covering method thus consists of regrouping signals for which the mid points of the segment connecting the source and the receiver are identical.
This regrouping is carried out in a purely fictitious manner by suitable selection of values of corresponding registered data. For obtaining good results, the operation of this method of multiple covering processing presupposes the perfect positioning of the source and of the receiver as well as the validity of the theoretical notion of the mid point. Very significant difficulties in the perfectly exact positioning of the source and of the receiver can appear, particularly in the case of seismic surveys obtained by marine prospecting when the or each of the receivers used are secured to a floating cable towed by a boat. Further, the notion of the mid point appears valid only in rare situations, the notion of the mid point being invalidated in the case where the mirror image point of reflection of the pressure waves considered has a slope or dipping.
Although there has at present been proposed a dipping correction by applications of predetermined space and time corrections by means of simple mathematic functions, these cannot pretend to permit obtaining specific results at the reflection point considered.